This invention is related to RF transponders and, in particular, to RF air coil transponders and to methods of making the same.
In radio frequency identification systems, the resonant frequency of the transponders is one of the most important factors in the reading performance of the transponders. Precise resonant frequency of the transponders helps to guarantee the electrical functionality quality of the product.
Often, some transponders demonstrate different reading performance and this difference often comes from differences of the resonant frequencies. This, in turn, is often caused by the technology used. For example, for low frequency transponders, such as 125 kHz transponders, and middle frequency transponders, such as 13.56 MHz transponders, usually air coil technology is used.
Generally, in the manufacture of air coil transponders, the air coils are made from conventional single insulated wires, which typically are self-adhering. If self-adhesive wires are used, this could result in the coils being solid and difficult to be deformed if the number of windings is high. On the other hand, if the number of windings is low, such as 1 to 50 turns, the coil could be fragile and easily deformable. If the coil is deformed, impedance of the coil is also changed.
Further, every coil comprises inductance, resistance and parasitic capacitance. If the distance between wire turns and the wiring process differs from one transponder to another, the internal parasitic capacitance will also be different, resulting in differences between the impedances of the coils. This, in turn, results in a difference in resonant frequencies.
It is known to adjust the resonant frequency of transponders by changing the capacitance and/or changing the inductance of the resonant circuit of the transponders.
A very high Q (quality factor) resonant circuit is particularly effective in capturing high energy from the reading device and re-transmitting energy to the reading device, particularly at longer reading ranges. On the other hand, a high Q circuit does not allow for wide tolerances of resonant frequency because frequency differences create high differences on the coupling of energy from the reader to the transponder and transmit less energy from the transponder to the reader. This not only reduces the maximum reading distance but also results in variations of maximum reading distance between transponders.
For this reason, more precise resonant frequency is necessary for a high Q resonant circuit maximize the reading distance and to minimize the differences between transponders.
In addition to problems of controlling resonant frequency due to variations caused by differences between transponder coils, problems can also result from variations in the associated electronic components. Thus, for example, the ICs (integrated Circuits) employed, have capacitors that affect the resonant frequencies and tolerance variations between the capacitor ICs, therefore, will cause variations in resonant frequency of the transponders.
The variations in coil electrical parameters and/or variations caused by electrical component tolerances necessitate an effective technique for adjusting resonant frequencies before during or after manufacturing.